So many questions. I will try to not to burden you with long responses.

1. it's actually not clear which language is, definitively, the oldest because, if you go far enough back, the line between images and language blur and it just becomes too hard to say. For example, look up some of the stele and reliefs from Gobekli Tepe. It's clear there are depictions of animals, but on some of the monuments there are more than one animal. Is this a story? Its impossible to know. The Lascaux caves is another good example. Does a language really need to have characters that represent sounds/words? Or are a series of images that tell a story sufficient? I think you see the problem.

But, I will answer your question. The oldest language, that you and I would agree is a language, (as far as we know) is Sumerian and other Elamite languages (there are a few languages from this time period). These are the oldest that we can read (sort of) but there are also other languages from this period that we can't and may not ever. After that the next oldest languages are the Afro-Asiatics (Egyptian, Semitic) which we can read and many more that we can't. Then you got Linear A and Linear B (probably the next oldest). Linear A is still indecipherable, but our boy, Michael Ventris, dedicated his life to crack B which we now know is the first manifestation of written Mycanaean Greek (but this form was not the same script that the Mycanaean kingdoms used later). Good stuff.

The oldest extent languages are mostly written in stone and clay. This, as you guessed, is because they stand the test of time more easily than woodcor metals. But there were many mediums onto which the ancients wrote including dirt, sand, clay, rocks, papyrus and leather (which I am using as a catch all term), metal, etc. Many of these media just disintegrate easily over time.

2. Yes. As I alluded to previously, there are a lot of them. In reality, we could probably read many more of they currently-undeciphered languages now if we just knew what language group it belonged to! Linear B is a good example for this. Found on the island of Crete, Ventris tried imposing many languages into the script before he finally realized archaic Greek fit! I mean seriously. In fact they still weren't sure if the code he cracked just fit the bill or was forced to fit until they found new tablets which were not used to produce the script and, low and behold, they could read them! It's honestly just incredible. Linear B is clearly taken from Linear A. The two scripts are very similar but they are clearly different languages. We still can't read Linear A and many poor academic souls have been lost in the pursuit. If only we could find more Rosetta stones...

3. Paper. Ok, this depends on how you define paper. It's actually a very important point. Paper, the word itself, comes from papyrus which is understood as a sheet of reeds woven together. Papyrus dates all the way back to the fourth millennium BC (!) and quite possibly even further. If you accept papyrus as paper, it is probably the oldest. However, if you understand paper to be unwoven, like today's paper, then it was the Chinese who probanly made it in the 1st century AD.

4. Yes! 100% true. In fact the Dead Sea scrolls were recently analyzed to help jigsaw them back together. It's really quite ingenious: by identifying the DNA of the sheep upon which a given scroll was written, they could at least determine which fragment more probably went with each other. During the analysis they found the scrolls to be written on the hides of many a sheep and even cows. This is just an old example, but I assure you that many manuscripts and codeces have been written on cured animal skins (even the rare human skin!).

Hope this helps. F